Van Gogh and Britain is a superb show with a misleading title conjuring images of Vincent in Blighty. That he did have a life here - in Ramsgate, then Brixton - is very nearly as surprising as the fact that Canaletto spent eight years in Soho. But it is our response to him that's at issue, and not vice versa, in particular our notorious resistance to his genius. Where continentals were buying his paintings not long after he died, it seems we remained indifferent or even found them (as late as a 1923 Bateman cartoon) simply hilarious.

That, at least, is the usual tale. In fact, as this show reveals, one brave Scot bought a van Gogh directly from him - a basket of apples, surging like a life raft on a sea of brushstrokes: so dynamic the canvas is practically bursting. The owner's horrified father immediately disposed of it for a fiver, poor fool, though the artist himself got nothing but the price of the fruit.

Three years after van Gogh's suicide in 1890, the British consul in Amsterdam bought what must be the wildest painting of crustaceans in all art - an utterly bizarre portrait of two raging-red crabs thrashing against an arsenical green ground, as if protesting their freedom. Stranger than Dali's lobster telephone, it still looks staggeringly avant-garde today and it's just as well the consul died two years after selling it for £8, avoiding a lifetime's remorse. For it soon changed hands for a fortune and now belongs to the National Gallery.

Not that the gallery makes much of it. Indeed, one of the thrills - there's no other word for it - of this compact show is that it is such a weird and unexpected selection. While the Europeans were snapping up sunflowers and starry nights, the British often bought darker and more complicated works. It still seems remarkable that a teetotal Calvinist-Presbyterian Welsh spinster should have sunk her funds into a very late painting like Rain, with its crouched houses and foreboding crows, in 1920. Looking at the lines streaking down this canvas - astonishing striations, Jackson Pollock far in advance - you realise that for van Gogh rain was in some sense comparable to drawing itself: delineating air, notating the landscape.

So it goes on: the marmalade manufacturer who bought shivering harvest fields; the stockbroker who had the wit to collect those dark and knotted drawings of peasants and cottages made when van Gogh was a young autodidact in his father's gloomy house. These exceptional images, with their bent, humbled figures, their exposed paper and delicate wintry branches, so much like Hiroshige's cherry trees, show how early and swiftly he absorbed the lessons of Japanese art.

Indeed, if you think of him as sui generis, then this exhibition shows you that he started out as one of a crowd. Some of the early scenes - restaurants on the Seine, women shaded beneath parasols - show direct descent from Monet and Pissarro. But just how far he got in his desperately short career soon becomes wonderfully apparent and it is an evolution you wouldn't so clearly discern in a more conventional outsize blockbuster.

It's a mere few years from those labourers - the very emblem of straitened fatigue, hunched over blackened fields, made while he was still teaching himself to draw - to the blazing glory of a masterpiece like Oleanders. Dawn-pink flowers glowing against verdant green, a golden copy of Zola's La joie de vivre placed so appositely, so ceremonially, beside them, this is the work of one of the greatest colourists of all time.

And soon the radicalism will reach its pitch - with those whirling skies and crackling trees, those electrifying brushmarks that channel the flow of sensation, still dazzling with passion, still live with the maker's mark - and soon he will die.

Van Gogh was 37 when he shot himself. Adam Elsheimer was dead at 32, possibly as a result of being imprisoned by his own pupil, a parasite who grew fat on the proceeds of etchings he made of the master's most famous works. If you've never come across Elsheimer, a German contemporary of Caravaggio and Rubens, you can see more or less everything he ever made in a brilliantly curated show at the Royal Scottish Academy. Working fiendishly slowly, Elsheimer painted in oil on small copper panels so densely crammed with landscape and narrative and astonishing light effects that Rubens declared himself heartbroken at his death. His masterpiece - just over a foot wide - is The Flight into Egypt, in which Mary and Joseph are nearly lost in the inky night between two pools of brilliance, a distant fire and the moon reflected in water.

Much is made of Elsheimer's extraordinary eclecticism: the way he introduces a Tintoretto angel or a Durer landscape, compresses Caravaggio's raking light into a few startling inches. Despite the scale, he is never cute. The tiny ladder in Jacob's Dream may ascend to a Tinkerbell heaven, but the dreamer is hardly sweet.

Actually, it is the singularity of Elsheimer's vision that strikes. The way Christ's baptism is crowded with busy angels and fancy-dress potentates. The heft of a big fish clutched under a little boy's arm. The way the torturer stoning a saint rises with professional pride to the challenge.

I love a painting of St Joseph, infinitely patient, keeping in painfully slow step with the toddler Jesus. The picture is no bigger than a football card but the saint is magnificent in context. Elsheimer wants you to be intimate with his characters, so tenderly drawn, which is achieved not just through scale but empathy.

Elsheimer lived in the age of Galileo. Surely there are no better paintings of stars than those in The Flight into Egypt with their cold, electric aura and queer brightness when seen through leaves. Something of their radiance may come from painting on copper, with its glow, but Elsheimer had a vision of the future. Look at his image of St Lawrence alone on a blasted heath, darkness gathering, and you see German Romanticism two centuries early. Look at his landscape before dawn, when the gold glimmering beneath the horizon touches the infinite clouds above and you see the sublime yearnings of Caspar David Friedrich.